Apparatus for the efficient use of ionizing radiation produced by microwave linear accelerators



April 5, 1960 D- R. DEWEY ll, EI'AL APPARATUS FOR THE EFFICIENT USE OFIONIZING RADIATION PRODUCED BY MICROWAVE LINEAR ACCELERATORS Filed Jan.31, 1955 3 Sheets-Sheet 1 4 FIG.

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2,931,941 RADIATION RATORS 3 Sheets-Sheet 2 DEWEY ll, ETAL IENT USE OFIONIZING MICROWAVE LINEAR ACCELE FIC Apnl 5, 1960 APPARATUS FOR THE EFPRODUCED BY Filed Jan. 31, 1955 MOD.

Apnl 5, 1960 D. R. DEWEY u, ETAL 2,931,941 APPARATUS FOR THE EFFICIENTUSE OF IONIZING RADIATION PRODUCED BY MICROWAVE LINEAR ACCELERATORSFiled Jan. -31, 1955 3 Sheets-Sheet 3 FIG. 4

United States Patent APPARATUS FOR THE EFFICIENT USE OF IONIZ- INGRADIATION PRODUCED BY MICROWAVE LINEAR ACCELERATORS Davis R. Dewey II,Lincoln, and John C. Nygard, Waltham, Mass., assignors to High VoltageEngineering Corporation, Cambridge, Mass., a corporation ofMassachusetts Application January 31, 1955, Serial No. 484,940 2 Claims.(Cl. 315-39) This invention relates to microwave linear accelerators,and in particular to novel apparatus for the eflicient use of theionizingradiation, including high-energy electrons and X-rays, producedby microwave linear accelerators. I 1

High energy electrons are one of the major forms of ionizing radiationused for sterilization of food, drugs,

and other materials, and for the promotion of chemical reactions. Themajor limitation attendant upon highenergy electrons is the relativelyshort range of electrons in matter. Thus a 2-m.e.v. electron has a rangeof about 1 cm. in water. The range is approximately proportional to theenergy of the electrons, so that increased penetration in the matterirradiated is obtained by increasing the electron energy.

Owing to the high energies attainable therewith, microwave linearaccelerators are a preferred source of electrons for these purposes.However, despite the fact that electron energies of 50 m.e.v. and highermay be obtained from microwave linear accelerators, the cost of amicrowave linear accelerator increases roughly in proportion to thesquare root of the power output. For eflicient operation at higherelectron energies, more power must be delivered to the microwave linearaccelerator than at lower electron energies, and the cost of theaccelerator per m.e.v. of electron energy increases with increasingelectron energy.

Moreover, at electron energies in excess of a certain energy, nuclearreactions may be produced in the matter irradiatedj The energy abovewhich such reactions may I occur varies, depending upon the particularmatter being Accordingly, one ob ect of our invention is to increase theefiective penetration in matter obtainable with electrons of a givenenergy, through the use of a plurality of essentially opposed electronbeams, and our invention comprehends novel apparatus for the productionof a Qplurality oi electron beams by means of microwave linear raccelerator components.

Another object of our invention is to increase the efficiency ofoperation of microwave linear accelerators Ewhich are used for theproduction of X-rays for the treatment-of patients in cancer therapy andthe like.

Our invention may best be understood from the following description withreference to the accompanying drawings in which, since the constructionof microwave linear accelerators is well-known in the art, the variouscomponents of microwave linear accelerators are merely diagrammaticallyillustrated.

velocity of the wave.

2,931,941 Patented Apr. 5, 196i) In the drawings: a

Fig. l is a diagram illustrating the major components of a conventionalmicrowave linear accelerator;

Fig. 2 is a diagram illustrating apparatus for the production, inaccordance with our invention, of two electron beams by means ofconventional microwave linear accelerator components;

Fig. 3 is a diagram illustrating a modification of the apparatus of Fig.2, in which three electron beams are produced by means of conventionalmicrowave linear accelerator components;

Fig. 4 is a diagram illustrating the irradiation of tubular materialwith four electron beams; and

Fig. 5 is a diagram illustrating apparatus for the treatment of patientswith X-rays produced by conventional microwave linear acceleratorcomponents, but wherein said components are arranged, in accordance withour invention, so as to increase efiiciency of operation.

Referring to the drawings, and first to Fig. 1 thereof, in a microwavelinear accelerator an electron beam 1 is created by an electron injector2 and injected axially into one end of a waveguide 3 in which atraveling electromagnetic wave is produced by means of highfrequencypower fed into the waveguide 3 from a highfrequency oscillator 4 such asa magnetron or klystron or other high-frequency oscillator. Theapparatus is designed so that the electromagnetic wave has an axialelectric field component, and the waveguide 3 is iris-loaded so that thephase velocity of the traveling wave is very nearly equal to thevelocity of light in vacuo, or 3 x 10. chm/sec. The electrons areinjected into the waveguide 3 at a velocity which is almost equal to thevelocity of light, so that the electrons start traveling down thewaveguide 3 with falrnost the same velocity as the phase Some of theelectrons will find themselves in an accelerating electric field whichraises their velocity to a value very nearly equal to the velocity oflight. These electrons can then gain only a negligible amount ofadditional velocity, and so they remain in the I L E +1; EdL electronvolts where E is the energy of the electrons at injection into thewaveguide 3.

The function of the oscillator 4 is to convert D.-C. power intohigh-frequency power. Owing to the high power involved, the D.-C. poweris not applied continuously to the oscillator 4, but rather in the formof square-wave pulses which are produced by a modulator S. The modulator5 is thus the basic power supply for the microwave linear acceleratorand furnishes a pulsed D.-C. output. The oscillator 4 converts thepulsed D.-C. output of the modulator 5 into high-frequency power, and inthe waveguide 3 the high-frequency output of the oscillator 4 isconverted into kinetic energy of the electrons injected into thewaveguide 3 by the injector 2. Since the traveling electromagnetic waveexists in the waveguide 3 only during the intervals in which themodulator 5 is delivering a square-wave pulse, the injector 2 is usuallypulsed in synchronism with the modulator 5 in order to conserve thepower required for the injector 2.

At the present time, a representative modulator 5 may give an output ofsquare-wave pulses about 1 to 10 or more microseconds in duration with apulse repetition rate of about 1 to 1000 pulses per second. Since thefrequency of the electromagnetic oscillations produced in the waveguide3 by the oscillator 4 is usually, at the present time, about 3000megacycles, the pulse length is seen to be very long compared with theperiod of the high-frequency oscillations. In describing the operationof microwave linear accelerators, reference is made throughout thedescription herein to conditions during the pulse. Thus, for example, atthe present time, magnetrons'can be constructed to give a power outputof up to 5 megawatts (during the pulse) and klystrons can be constructedto give a power output of up to megawatts (during the pulse) 5 but theaverage output of such magnetrons and klystrons is less than the figuresmentioned. V I

The power output of fa microwave linear accelerator is limitedprincipally by the oscillator 4. Thus, as stated, the power output of amagnetron oscillator is limited to about 5 megawatts at the presenttime, and the power output of a klystron oscillator is limited to about10 megawatts at the present time. The lr gh-frequency power output ofthe oscillator may beincreased to means of one or more amplifiers, suchas klystrons. In general, only one oscillator is used, owing to thedifficulty "of properly pha'sing the outputs of more than one oscillaterin order to produce the desired traveling wave in the waveguide 3. V

As stated, the basic power source for the microwave linear acceleratoris the modulator 5, and in general the modulator 5 may be designed "togive 'sufiicient power for the operation of any present day oscillator,with or without amplifiers, i v v c p The frequency of thehigh-frequency power delivered by the oscillator 4 to the waveguide 3should be as high as. possible. A frequency as high as 10,000 megacycleswould be desirable, but the power tubes currently available limit thefrequency attainable. At the present time, a frequency of 3,000megacycles may be taken as representative, which corresponds to awavelength of 10 cm. for an unbounded wave in vacuo.

The length of the waveguide 3 is determined by the electron energydesired. For a given waveguide 3 of length L there is a maximum electronenergy attainable for a given power input which'is greater the greatervalue of L. However, increas ng the length L of the waveguide 3decreases the 'efiiciency of the microwave linear accelerator, Hencethe-length L of the waveguide 3 should be great enough to providethedsired electron energy, but no greater.

Moreover, the electron energy decreases as the electron beam currentincreases, and the maximum electron energy just referred to is theelectron energy approached as the electron beam current approaches zero.Therefore, the length L of the waveguide 3 must be great enough toprovide the desired electron energy at the desired electron beamcurrent.

The foregoing discussion is summarized by the following approximateequation:

-1 is the'electron ssamf -c ags linainpe'rs') L is the length of thewaveghide fincm.)

a is the attenuation of thewaveguide (in -nepers7n1i) P is thehigh-frequency power delivered to 'the waveguide by the oscillator (inwatts) A is a constantfor agiven waveglideconstructionand a givenfrequency (in Vohms/m'.)

a It is apparent from the foregoing equation that, as the electron beamcurrent I increases, the electron energy decreases linearly from amaximum of It is also apparent that for a given length L of thewaveguide, the power P =l V in the electron beam is a maximum, and hencethe efficiency of the microwave 10 linear accelerator is a maximum, whenThe foregoing equations are only approximate, and in practice themicrowave linear accelerator is usually operated with a beam currentsomewhat lower than that given by Equation 4. These equations show that,for a given length L of the Waveguide S and when V and I 5 have thevalues given by Equations 3 and 4, the electron 56m nqiiati'e'n an aparent that the eflicienc'y appreaches 100% as 'aL becomes very -small,and that the 40 as aL becomes very large. Evidentlyit is desirable tominimize the length 'L ofthe waveguide, "as hereinbefore stated. FromEquation 3 it appears that the lower limit of L for ".ag'iven microwavelinear acceleratoris imp'o'sed'by the upper limit of the poweravailable. Hence,

if the available power is limited, the efficiency is also 7 Mareever,Equations '3 and 5 show that'if the electroh energy V 'is increased, thepower input P must increase in proportion to V? or 'else theeffi'cien'cy is lowered. 'Ifthe power input P is not increased inproportion "to V the efficiency is lowered because either the energyV'will not have its most efiicient value (i.e. /.,.V or else 'L must beincreased.

The'efiiciencies given by Equation 5 for various values of aL are shownin the following table:

Table] uL Eiliclency L if a='.004 nepers/cm.

Percent cm.

NorErfAt'the present time,-waveguide lengths of less'fihan' 50 cm; are 2not practical owing to the excessive amount gi power required toproducethe desiredielectron energy. Moreover, even if the very high powerrequired were attainable, the microwave linear accelerator would ha've(F0. operate at yeryh gh currents in iqrder to operate.efliciently. Thiswould create difficult problems or electron emission and bemlocusing.

'to the object 6 is proportional to the electron beam current I. Whileincreasing the beam current I decreases the length of time required todeliver the desired dose, the magnitude of the beam current I is lesscritical than that of the energy V, which obviously must be suflicientlygreat or else part of the object 6 receives no dose at all.

It is well-known in the art that the electron energy required for fullpenetration of a given object may be reduced by irradiating the objectfrom opposite aspects with two opposed electron beams. Using thisdoublebombardment technique, the energy required is at most half thatrequired with single bombardment, and usually the energy. required indouble bombardment is less than half that required in singlebombardment, owing to the distribution in depth of the ionizing energyof the electron beam.

Our invention comprehends novel apparatus for the production of two (ormore) electron beams for the purpose of irradiating objects with highenergy electrons using double-bombardment techniques. Our inventionprovides advantages over single-bombardment techniques which are commonto other double-bombardment techniques. If an object of thickness t isirradiated with single-bombardment techniques, the electrons must havesufficient energy so that an adequate ionizing dose is delivered at thedepth t. However, in order that enough electrons may reach the depth t,the beam energy must be so high that some of the electrons travelthrough the object, with the result that some of the beam power iswasted. Using double bombardment, this excess beam power is delivered tothe interior of the object, and is not wasted. Hence double bombardmentmakes more efiicient use of the available beam power.

By our invention, not only is the increased efiiciency inherent indouble-bombardment techniques obtained, but the conversion ofhigh-frequency power to electron beam power is also more eflicient thanin the single-bombardment case, as will appear hereinafter.

Referring now to Fig. 2, therein is shown apparatus for producing twoopposed electron beams in accordance with our invention. Electron beams7 and 7 are created by electron injectors 8 and 8' and injected intowaveguides 9 and 9' respectively, and after being accelerated theelectron beams 7 and 7 impinge upon the object 6 from opposing aspects.The high-frequency power for the waveguides 9 and 9 is derived from theoscillator 4, which is driven by the modulator 5. The high-frequencyoutput of the oscillator 4 is divided equally by a microwave powerdivider 10, so that one-half of the power output of the oscillator 4 isdelivered to each waveguide 9, 9'.

In order to compare the apparatus of Fig. 2 with that of Fig. 1, it willbe assumed that the oscillator 4 and the modulator 5 are the same inboth cases, that the electron injectors 8 and 8 are each identical tothe electron injector 2, that the object 6 is the same in both cases,and that the waveguides 9 and 9 are each identical to the waveguide 3except for their lengths.

Referring first to Fig. 1, the electron energy V required to irradiatethe object 6 adequately is determined by the effective thickness t ofthe object 6. Using the double bombardment arrangement of Fig. 2, theelectron energy V required of each beam 7, 7 to irradiate the sameobject 6 is, as hereinbefore stated, therefore somewhat less that /2\',,but for simplicity of discussion it will be assumed that V /2V If P isthe power output of the oscillator 4, the power delivered to thewaveguide 3 of Fig. 1 will be P, and

the power delivered to each of the waveguides 9 and 9' of Fig. 2 will be/2P. The appropriate lengths of the waveguides are then given byEquation 3, as follows:

For waveguide 3 of Fig. 1:

From Equation 8 it is seen that the length L of each of the waveguides 9and 9 is less than the length L of the waveguide 3. Consequently, theconversion of highfrequency power to electron beam power is moreefficient in the case of the apparatus of Fig. 2 than in the case of theapparatus of Fig. 1. This means that the beam current 1 delivered byeach of the waveguides 9, 9' of Fig. 2 is greater than the beam currentI delivered by the waveguide 3 of Fig. 1. v A

The foregoing may be illustrated by a specific example. Suppose that,for both the apparatus of Fig. 1 and that of Fig. 2:

a=.004 neper s/cm. A=60 /ohm/cm. P=4 megawatts If the thickness t of theobject 6 is such that the energy of the electrons produced'by theapparatus of Fig. 1 must be 10 mev., Equation 6 shows that the length Lof the waveguide 3 will be 275 cm. Table I shows that the efiiciency atthisllength is about 50%, and the beam current I, will therefore be .2ampere.

From Equation 8, it is seen than the length L of each of the waveguides9 and 9 will be cm. Table I shows that the efficiency at this length isabout 70%, and the beam current I will therefore be .56 ampere.

From Table I it is apparent that, as greater lengths are reached, theefiiciency becomes greatly reduced. From Equation 3 it is apparent that,for a given energy, the limitation upon the shortness of the waveguideis the maximum power available. As stated previously, the power limit isimposed mainly by the oscillator, although amplifiers may be used toincrease the power output of the oscillator. Owing to the power limit,the efficiency becomes increasingly less as higher electron energies arerequired. As stated, one of the advantages of our invention is the factthat this reduction in efiiciency at the higher energies is overcome.

In some cases, such as where particularly thick products are to beirradiated, more than two electron beams may be desired, and ourinvention also comprehends the use of more than two waveguides toproduce the desired number of electron beams. In Fig. 3, three electronbeams 11, 11', 11" are created by electron injectors 12, 12', 12 andinjected into waveguides 13, 13', 13" respectively, and after beingaccelerated the three electron beams 11, 11', 11 simultaneously bombardan object 14 from essentially opposing aspects.

Where greater power is required than is available from a singleoscillator, the output of the oscillator may be amplified, ashereinbefore stated, by amplifiers. Alternatively, more than oneoscillator may be employed. Thus, in the apparatus of Fig. 3, a separateoscillator 15, 15, 15 is used for each waveguide 13, 13', 13" and asingle modulator 16 drives all three oscillators 15, 15, 15". Generallythe output stage of the modulator 16 comprises a pulse transformer whichtransforms a relatively lowvoltage pulse into the desired high-voltagepulse. In the apparatus of Fig. 3, wherein three oscillators areemployed, it is preferable to provide a pulse transformer 17, 17', 17"ror each oscillator, the pulse transformers 17,

7 17', 17" beingplaced near their respective oscillators 15, 15', 15".The relatively low-voltage output of the modulator 16 is then divided bya suitable power divider 18 and applied to the 3 pulse transformers .17,17, 17". This arrangement minimizes the undesirable effect ofcapacitance between the relatively long leads 19, 19', 19" and ground,by sending low-voltage pulses rather than high-voltage pulses alongthese'long leads. Since each oscillator drives a difieren't waveguide,there is no problem of phasing their outputs.

The irradiation of an object by a plurality of electron beams isadvantageous, not only for the purpose of irradiating thick objectsQbutalso for the purpose of irradiating tubular material, particularly ifthe tubular material to be irradiated encloses a high-density materialwhich need not be irradiated but which would absorb much of theionizing-energy of a single electron beam, since the electron beam wouldhave to pass through the high-density material in order fully toirradiate the tubular material. For example, it may be desired toirradiate a length of extruded polyethylene tubing (or other extrudedplastic tubing) with high energy electrons in order to causecross-linking of the polyethylene (or other plastic), or it may bedesired to irradiate a length of cable, comprising a conductor such ascopper surrounded by a sheath of polyethylene (or other plastic), forthe same purpose. In either event, it is desired to irradiate a layer ofannular cross-section. This may conveniently be done by using severalelectron beams, as shown in Fig. 4, wherein four electron beams 20, 20,20", 20" are sirnultaneouslydi- .rected upon the circumferential surfaceof a length of :26, 26', wherein patients, situated as at 27, 27',receive X-ray treatment in cancer therapy or the like. paratus of Fig..is similarto that of Fig. 2, except that The apthe output of thehigh-frequency power unit is applied only to one waveguide'at a time, bymeans of a suitable -microwa've power switch 28.

In X-ray installations for treatment of patients, the X-ray apparatus isnot operated during the time in which each patient is properlypositioned in the treatment room. Generally the patient-preparation timeis about twice the actual irradiation time. Hence, a given installationis actually operated only about one-third of the available time.

By means of the arrangement shown in Fig. 5, the

patient-treating capacity of a microwave linear accelerator installationis doubled at an increase in capital cost which is only a small fractionof the total cost of a complete new installation. While our inventionincludes the use of a single high-frequency power unit to drivealternately more than two waveguides in a manner similar to that shownin Fig. 5, it would probably not be advantageous toprovide .more thanthree waveguides for a singlejpower unit, since, as hereinbefo-restated, each waveguide would be in operation about one-third of--theavailable time. v

In the foregoing description, particular reference is made totraveling-wave linear accelerators, since such linear accelerators areincommon use. However, it is evident that the-advantagesof our inventionapply equally well to standing-wave or resonant-cavity linearaccelerators, and that our invention is not limited to any particulartype of microwave linear accelerator.

Having thus described several illustrative embodiments of our invention,his to be understood that although specific terms are employed, they areused in a generic and descriptive sense and not for purposes oflimitation, the scope of the invention being set forth in the followingclaims:

We claim:

1. Apparatus for irradiating matter with high-energy electronscomprising in combination a plurality of electron accelerator units,each unit including a waveguide and means'for injecting electrons intothe waveguide, each unit being adapted to accelerate an electron beamupon excitation of the waveguide by a high-frequency power source; ahigh-frequency power source whose power out- .put is suitable for theexcitation of said waveguides;

means for delivering approximately half of the power output from saidhigh-frequency power source simultaneously to each of said waveguides;and means for directing the electron beams produced by said waveguidesonto matter to be irradiated from essentially opposing aspects.

2. Apparatus for irradiating matter with high-energy electronscomprising in combination a plurality of electron acceleratorassemblies, each assembly including a waveguide adapted to accelerateelectrons so as to produce an electron beam upon being excited byhigh-frequency power, means for injecting electrons into the wave,guide, and a high-frequency oscillator adapted to convert D.-C. powerdelivered thereto into high-frequency power and to deliver suchhigh-frequency power to the waveguide; a power source providing a DC.power output suitable for energizing said oscillators; means fordelivering approximately half of the power output from said power sourcesimultaneously to each of said oscillators; and means for directing theelectron beams produced by said waveguides onto matter to be irradiatedfrom essentially opposing aspects.

References Cited in the file of this patent UNITED STATES PATENTS

